1. Field of the Invention
The present invention relates to an ISDN (Integrated Service Digital Network) exchange or a data terminal control unit such as of an ISDN key telephone system.
2. Description of the Background Art
FIG. 9 shows structure of a basic interface of ISDN and a plurality of ISDN data terminals which are multidrop-connected to the basic interface. A communication circuit network 20 is connected via a subscriber's digital service unit (DSU) 21 to the subscriber's data terminals 1, 2, 3, 4 and so on. FIG. 10 is a diagram of enlargement of subsequent structure to DSU 21.
In ISDN, processings of originating calls and terminating calls are carried out by employing a massage called "setup message" at the time of originating and terminating. An area storing the setup message therein includes a region for storing an ISDN address therein. This ISDN address includes an originating ISDN address for designating an address of an originating data terminal and a terminating ISDN address for designating an address of a terminating data terminal. The ISDN address is further divided into an ISDN number and an ISDN subaddress.
In general, when a setup message is transmitted to a terminating data terminal, network 20 does not set a terminating ISDN number. As one of services conducted by network 20, however, there is a direct dial-in service. In this service, a number set to a terminating ISDN number of a setup message which is transmitted to network 20 by an originating terminal is set as a dial-in number in a terminating ISDN number of a setup message which is transmitted to a terminating terminal by the network. Accordingly, if a plurality of data terminals are multidrop-connected to a single interface as shown in FIG. 4 or 5, each data terminal can recognize whether or not the transmitted setup message is destined for itself by use of the dial-in number included in the terminating ISDN number.
When data terminals capable of recognizing ISDN numbers are connected to a single interface, these terminals store different dial-in numbers therein. As shown in FIG. 10, assume that respective data terminals 1-3 having, e.g., respective dial-in numbers 123451, 123452 and 123453 are connected via DSU 21 to network 20. When communication is intended to be made with data terminal 2 having dial-in number 123452, the originating data terminal sets dial-in number 123452 in a terminating ISDN number and transmits a setup message SM. Data terminal 2 having dial-in number 123452 refers to terminating ISDN number 123452 of transmitted setup message SM and determines that the transmitted setup message is a self-destined setup message.
Consideration is then made on a case where the number of interfaces on extension side is increased by connecting an ISDN exchange including a DSU or a data terminal control unit 22 including an ISDN key telephone system main apparatus between network 20 and plural terminals 1-9, as shown in FIG. 11. Referring to FIG. 11, a port 31 on extension side is connected with respective data terminals 1-3 having respective dial-in numbers 123451, 123452 and 123453, a port 32 with respective data terminals 4-6 having respective dial-in numbers 123454, 123455 and 123456, and a port 33 with respective data terminals 7-9 having respective dial-in numbers 123457, 123458 and 123459. Those ports are connected to data terminal control unit 22. When a terminal on outside-line side intends to communicate with data terminal 2 having dial-in number 123452 of port 31, the outside-line data terminal sets dial-in number 123452 in a terminating ISDN number and transmits setup message SM. Data terminal control unit 22 which has received setup message SM from network 20 refers to the terminating ISDN number of setup message SM and determines that data terminal 2 of dial-in number 123452 is connected to port 31, thereby to transmit setup message SM of terminating ISDN number 123452 to port 31.
Similarly, assume that extension terminal 8 (dial-in number 123458 of port 33) originates a call to extension data terminal 2 (dial-in number 123452 of port 31). Originating data terminal 8 sets dial-in number 123452 in a terminating ISDN number and transmits setup message SM. Data terminal control unit 22 determines a port number on the basis of the terminating ISDN number of the received setup message SM and transmits setup message SM of terminating ISDN number 123452 to port 31. In such a manner, identification of the terminating data terminal is carried out in accordance with the contents of the ISDN number set by the originating data terminal.
As described above, when communication is made by identifying a single terminal which is multidrop-connected to a single interface, it is possible to use a dial-in number set in an ISDN number. In this case, it is unnecessary to use an ISDN subaddress.
In general, when an extension data terminal calls another extension data terminal, if an extension number which is alloted to each extension data terminal by a number system independent of a dial-in number can be used, the number of digits of each number is decreased. This is therefore advantageous. In a conventional method, a dial-in number must be used even for connection from a certain extension data terminal to another extension data terminal.
In addition, when alteration is made on an extension number in a case where a dial-in number is used as the extension number, the dial-in number registered on an extension data terminal must be changed.
FIG. 12 is a diagram showing another example of the detail of a subsequent structure to DSU 21 of FIG. 9. When data terminals which can recognize ISDN subaddresses are connected to one interface, these data terminals store therein different ISDN subaddresses. As shown in FIG. 12, in a case where data terminals 1, 2 and 3 having respectively, subaddresses 11, 12 and 13 are connected via DSU 21 to network 20, if communication is intended to be made with the data terminal having subaddress 12, an originating data terminal sets subaddress 12 in a terminating ISDN subaddress and transmits a setup message SM. The data terminal having subaddress 12 refers to a terminating ISDN subaddress of transmitted setup message SM and determines that the message is a setup message destined for the data terminal itself.
Such a case is considered that the number of interfaces on extension side is increased by connecting an ISDN exchange or ISDN key telephone system main apparatus (hereinafter referred to as "data terminal control unit") 22 between network 20 and data terminals. First interface (a port P1) on extension side is connected with data terminals 1, 2 and 3 having, respectively, subaddresses 11, 12 and 13, a second interface (a port P2) with data terminals 4, 5 and 6 having, respectively, subaddresses 11, 12 and 13, and a third interface on extension side (a port P3) with data terminals 7, 8 and 9 having, respectively, subaddresses 11, 12 and 13. When an outside-line data terminal intends to communicate with data terminal 2 having subaddress 12 of port P1, the outside-line data terminal sets a value for identifying the port ("1" in this case) and subaddress 12 of data terminal 2 in a terminating ISDN subaddress and transmits a setup message SM1 (the value is "112"). Data terminal control unit 22 which has received setup message SM1 from network 20 refers to the terminating ISDN subaddress and transmits a setup message SM of terminating ISDN subaddress 12 to port P1.
Similarly, when extension data terminal 8 (the terminal of subaddress 12 of port P3) originates a call to extension data terminal 2 (the data terminal of subaddress 12 of port P1), (1) originating data terminal 8 transmits a setup message SM2 in which a port number 1 is set in a terminating ISDN number, and "12" in a terminating ISDN subaddress. Alternatively, (2) originating data terminal 8 transmits a setup message SM in which port number 1 and subaddress 12 are set in a terminating ISDN subaddress.
In the case (1), data terminal control unit 22 determines from the received setup message a port number on the basis of the terminating ISDN number, and transmits a setup message SM of terminating ISDN subaddress 12 to port P1. In the case (2), data terminal control unit 22 transmits setup message SM of terminating ISDN subaddress 12 from setup message SM1 to port P1. In the both cases, identification of a terminating data terminal is made on the basis of the contents of an ISDN subaddress set by an originating data terminal.
As described above, when one of data terminals which are multidrop-connected to a single interface is identified to make communications, an ISDN subaddress must be used. Accordingly, the originating data terminal sets an ISDN subaddress and transmits a setup message.
In general, however, when an extension data terminal originates a call to another extension data terminal, it is convenient to use an extension number allotted to each of extension data terminals. Since this corresponds to an ISDN number in ISDN, it is desirable to identify a terminating data terminal on the basis of the ISDN number in order to realize the same operating environment as that of a conventional data terminal control unit.
In addition, when extension numbers are changed with subaddresses used as the extension numbers, the subaddresses registered in extension data terminals must be changed.